Method and apparatus for forming laminated thin films or layers

ABSTRACT

Pre-coating films are formed in a pretreatment by supplying first film-forming gases into a process chamber of a process vessel while heating the process chamber so as to form a first pre-coating film on the inner surface of the process vessel exposed to the process chamber, followed by supplying second film-forming gases into the process chamber to form a second pre-coating film on the first pre-coating film. A semiconductor wafer is loaded into the process chamber. Then, the first gases are supplied into the process chamber while heating the process chamber so as to form a first layer on the wafer, followed by supplying the second gases into the process chamber so as to form a second layer on the first layer. A silane gas is supplied into the process chamber to permit silicon material to be deposited on the surface of the second layer stacked on the first layer. Finally, the wafer having the first and second multi-film is unloaded out of the process vessel.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for forminglaminated thin films or layers, i.e. a multi-layer structure, on asubstrate such as a semiconductor wafer or a glass substrate so as toform, for example, a gate electrode of an MOSFET.

In the general method of manufacturing a semiconductor integratedcircuit, a film formation on a substrate such as a semiconductor waferor a glass substrate and a patterned etching of the formed film arecarried out repeatedly so as to obtain a desired semiconductor element.

FIGS. 9A and 9B collectively show a conventional method of forminglaminated thin films collectively acting as a gate element of an MOSFETon a surface of a semiconductor wafer. As shown in FIG. 9A, an impurityof one conductivity type is diffused through the surface of the wafer Wto form a source region 2 and a drain region 4, followed by forming agate oxide film 6 consisting of, for example, SiO₂, on the wafer surfaceintermediate between these source and drain regions. As a result, achannel region is formed below the gate oxide film 6 such that thechannel region is sandwiched between the source region 2 and the drainregion 4. Further, an electrically conductive gate electrode 8 of amulti-layer structure is formed on the gate oxide film 6 so as toprepare an MOS transistor.

In general, the gate electrode 8 is not of a single layer structure. Inrecent years, the gate electrode 8 is of a two-layer structure in viewof, for example, an electrical conductivity of the electrode. In theprior art exemplified in FIGS. 9A and 9B, a polycrystalline silicon(polysilicon) layer 10 doped with phosphorus is directly formed on thegate oxide film 6. Further, a metal silicide layer, e.g., tungstensilicide layer 12, is directly formed on the polysilicon layer 10, asshown in FIG. 9A. Thus, the gate electrode 8 is of a double layerstructure consisting of the phosphorus-doped polysilicon layer 10 andthe tungsten silicide layer 12.

Recently, a semiconductor integrated circuit is made finer and finer toincrease the degree of integration. Naturally, requirements fordecreasing a working line width and a gate width are made severer andseverer. Also, a film thickness tends to be decreased to meet arequirement for an element of a multi-layer structure. Under thecircumstances, each of the independent layers, as well as adjacentlayers, collectively forming the multi-layer structure is required toexhibit electrical characteristics equivalent with or superior to thosein the prior art in spite of the decreased thickness of each of theselayers. The gate electrode 8 of the double-layer structure, which isshown in FIG. 9A, consisting of the phosphorus-doped polysilicon layer10 and the tungsten silicide layer 12 is intended to meet theabove-noted requirements.

It should be noted that a spontaneous oxide film 14 tends to be formedeasily on a surface of a silicon-based film, e.g., the phosphorus-dopedpolysilicon layer 10, upon exposure to the air atmosphere containingwater vapor, as shown in FIG. 9B. If the tungsten silicide layer 11 isformed directly on the oxide film 14, the bonding strength between thepolysilicon layer 10 and the tungsten silicide layer 12 is impaired. Inaddition, a sufficient electrical conductivity between these layers 10and 11 cannot be ensured, leading to deterioration in the electricalcharacteristics of the gate electrode 8.

In general, the polysilicon layer 10 is formed by a batch systemsimultaneously handling a large number of wafers in each lot, e.g., 150wafers. On the other hand, the tungsten silicide layer 12 is formed byone-by-one process. It follows that the many polysilicon layers formedon the wafers by the batch system differ from each other in the exposuretime to the air atmosphere and, thus, in the thickness of thespontaneous oxide film formed on the polysilicon layer 10. To overcomethis difficulty, a wet etching using, for example, an HF-based vapor isapplied to the surface of the polysilicon layer 10 so as to remove thenative oxide film 14.

However, even if the native oxide film is removed by a wet etchingimmediately before formation of the tungsten silicide film 12, it isvery difficult to prevent completely the base layer, i.e., polysiliconlayer 10, from being adversely affected.

A measure for overcoming the above-noted difficulty is proposed in, forexample, Japanese Patent Disclosure (Kokai) No. 2-292866. Specifically,it is proposed to form the phosphorus-doped polysilicon layer 10 withina process chamber, followed by consecutively forming the tungstensilicide layer 12 within the same process chamber.

Where the phosphorus-doped polysilicon layer 10 and the tungstensilicide layer 12 are consecutively formed within the same processchamber as proposed in the prior art noted above, it is certainlypossible to prevent a native oxide film from being formed on the surfaceof the polysilicon layer 10, making it possible to form the electrode 8exhibiting good electrical characteristics. In this case, however, a newproblem is brought about. Specifically, where, for example, 25 wafers ina single lot are successively processed, careful attentions must be paidto a thermal instability on the wall surface of the process chamber orwithin the wafer-processing apparatus including the process chamber aswell as to an instability in terms of the heat emission rate. If animpurity doped-polysilicon layer or the like is formed under thiscondition, the reproducibility of the formed layer is deteriorated.

Further, if the doped-polysilicon layer 10 and the tungsten silicidelayer 12 are formed successively, stress remains in the upper tungstensilicide layer, leading to deterioration in the bonding strength betweenthe tungsten silicide layer and the lower polysilicon layer. What shouldalso be noted is that, an annealing treatment is applied in general at,for example, about 900° C. after formation of the tungsten silicidelayer. In this annealing step, oxygen is diffused into the tungstensilicide layer 12 so as to deteriorate the electrical characteristics ofthe gate electrode 8.

Further, the reaction rate is limited in the reaction for forming apolysilicon layer; whereas, the reactant supply rate is limited in thereaction for forming the tungsten silicide layer. What should be notedis that the conventional shower head structure for introducing reactantgases for forming a film is incapable of coping with the above-noteddifference in the rate-limiting type. It follows that the gas streamfails to flow uniformly over the entire substrate surface in the step offorming any of the phosphorus doped-polysilicon layer and the tungstensilicide layer, with the result that the uniformity is impaired in thethickness of the formed film.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of forming afilm excellent in reproducibility of the film formation, which permitssuppressing stress remaining within the formed film and also permitspreventing oxygen from being diffused into the film under formation.

Another object is to provide an apparatus for forming a film, whichpermits coping with each of the reaction rate-limiting type reaction andthe reactant supply rate-limiting type reaction.

In the present invention, a plurality of pre-coat layers are formed onthe inner walls of each of the process chamber and the inner structureof the processing apparatus including the process chamber. The number ofthese pre-coat layers is equal to that of the films actually formed on asubstrate. Also, these pre-coat layers are formed before formation ofdesired films on the substrate. It follows that the internal environmentsuch as a heat emission rate from the inner wall of the process chamberor the inner structure of the processing apparatus is stabilized in thestep of actually forming the desired films on the substrate. Since thedesired films are consecutively formed on the substrate under thestabilized internal environment, the reproducibility of the filmformation can be markedly improved in the present invention.

Further, an after-treatment is applied to the formed film in the presentinvention. As a result, silicon is slightly attached to the surface ofthe formed film. What should be noted is that the attached siliconfunctions to moderate the stress imparted to the formed film, leading toan improved bonding strength between adjacent films formed on thesubstrate. Still further, even if a heat treatment is applied to thesubstrate during the after-treatment, the attached silicon serves toprevent oxygen from attacking the silicon such as polysilicon, making itpossible to prevent oxygen from being diffused into the formed film.

In the present invention, the film-forming step and the after-treatingstep are consecutively applied to a single substrate within a singleprocessing apparatus, followed by unloading the processed substrate fromthe processing apparatus. Then, these film-forming step andafter-treating step are consecutively applied to another singlesubstrate within the same processing apparatus, followed by unloadingthe processed substrate from the processing apparatus. In this fashion,a plurality of substrates, e.g., 25 semiconductor wafers, in a singlelot are consecutively processed.

After completion of the film formation process applied to apredetermined number of substrates, a cleaning gas is supplied into theprocess chamber for the cleaning purpose, followed by supplying a silanegas into the process chamber for performing a heat treatment. The heattreatment with a silane gas in the after-treatment for the cleaningpurpose makes it possible to lower the amount of halogen elementsremaining within the process chamber. This is effective for shorteningthe pre-coating time for the pretreatment step before formation of filmson the substrate.

The particular film formation process of the present invention can beemployed for depositing a tungsten silicide layer on a phosphorus-dopedpolysilicon layer so as to form a gate electrode of, for example, anMOSFET.

The apparatus of the present invention for forming a plurality of thinfilms includes a shower head section provided with a uniform dispersionplate having a large number of dispersion holes formed therethrough.

What should be noted is that the diameter and arranging density of thedispersion holes are set appropriately in the present invention. As aresult, the reactant gases for forming the desired films can be diffuseduniformly over the entire region within the process chamber in each ofthe step of supplying reactant gases for the reaction rate-limiting typereaction and the step of supplying reactant gases for the reactantsupply rate-limiting reaction. It follows that each of the films formedby the apparatus of the present invention is rendered highly uniform inthickness over the entire region of each film.

In the present invention, the uniform dispersion plate noted aboveconstitutes the uppermost stage of a gas spurting region. Also, thediameter of each of the dispersion holes is set to be at most 0.7 mm.Further, these dispersion holes are arranged at a density of at least0.3 hole/cm².

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a cross sectional view schematically showing the constructionof a film-forming apparatus used for working a film-forming method ofthe present invention;

FIG. 2 is a partial plan view showing a uniform dispersion platearranged within a shower head section of the film-forming apparatusshown in FIG. 1;

FIG. 3 is a flow chart showing the process steps included in a methodaccording to one embodiment of the present invention;

FIG. 4 is a graph showing the changes in the sheet resistance of eachwafer, covering the case where a film is formed on the surface of eachof the semiconductor wafers within first and second lots, each lotconsisting of 25 wafers;

FIG. 5 is a graph showing the degree of oxygen diffusion into the formedfilm, covering the cases where silicon is attached, and is not attached,to the upper surface of the base layer;

FIG. 6 is a graph showing the effect produced by an after-treatment stepin respect of moderation of stress remaining within the film formed inadvance;

FIG. 7 is a graph showing the remaining amount of the halogen elements,covering the cases where a cleaning after-treatment is performed, and isnot performed;

FIG. 8 is a graph showing the effect produced by a cleaningafter-treatment in respect of the pre-coating on, for example, the innerwall of the process chamber in preparation for the subsequent filmformation on a substrate; and

FIGS. 9A and 9B are collectively directed to formation of a gateelectrode in the conventional MOSFET.

DETAILED DESCRIPTION OF THE INVENTION

Let us describe an apparatus and a method for forming laminated filmsaccording to one embodiment of the present invention with reference tothe accompanying drawings.

Specifically, FIG. 1 is a cross sectional view schematically showing theconstruction of a film-forming apparatus used for working a film-formingmethod of the present invention. On the other hand, FIG. 2 is a partialplan view showing a uniform dispersion plate arranged within a showerhead section of the film-forming apparatus 16 shown in FIG. 1. Thefilm-forming apparatus 16 is of one-by-one processing type and isprovided with heating lamps which permit rapidly heating a semiconductorwafer.

The film-forming apparatus 16 comprises a process vessel 18 constructedby an upper wall, a circumferential side wall and a bottom wall. Aprocess chamber 20 is defined within the process vessel 18. Also, theprocess vessel 18 should desirably be cylindrical and is formed of ametal such as aluminum. A support cylinder 22 open at the upper andlower ends is formed coaxial within the process vessel 18. The lower endof the support cylinder 22 is fixed to the bottom wall of the processvessel 18. On the other hand, a plurality of support members 24 areequidistantly supported in the circumferential direction by the upperend of the support cylinder 22. In a preferred embodiment, three supportmembers 24 are arranged about 120° apart from each other. Each of thesesupport members 24 is in the shape of L consisting of a horizontal armportion and a vertical arm portion. The proximal end portion of thehorizontal arm portion is mounted to the upper end of the supportcylinder 22, with the distal end portion extending horizontal toward thecenter of the support cylinder 22. Further, the vertical arm portionextending upward is integrally fixed to the inner end of the horizontalarm portion. The upper ends of these vertical arm portions are connectedto peripheral portions in a lower surface of a disk-like susceptor 26.As a result, the support member serves to support the susceptor 26 suchthat the table 26 is concentric with the support cylinder 22. Asubstrate such as a semiconductor wafer W is disposed on the susceptor26. It should be noted that the diameter of the susceptor 26 issubstantially equal to or larger than that of the wafer W. Each of thesupport cylinder 22 and the support member 24 is formed of a heatray-transmitting material, e.g., quartz. Further, the susceptor 26 ismade of a material having a thickness of about 1 mm to 5 mm, andexhibiting a high resistance to heat and a good thermal conductivity.For example, the table 26 is made of a carbon-based material or analuminum compound such as aluminum nitride.

A plurality of L-shaped lifter pins 28 are arranged below the susceptor26. For example, 3 lifter pins 28 are arranged below the table 26. Theselifter pins 28 are joined to each other by a common ring (not shown) soas to be movable together in a vertical direction. Each of these lifterpins 28 consists of a vertical arm portion and a horizontal arm portion.The vertical arm portion is positioned below the susceptor 26 andextending in a vertical direction. On the other hand, the horizontal armportion horizontally extends outwards through the support cylinder 22.As pointed out above, these lifter pins 28 are movable in a verticaldirection. To be more specific, slots extending in a vertical directionare formed within the support cylinder 22. Of course, the horizontal armportion of the lifter pin 28 extends through the slot formed within thesupport cylinder 22. The upper end of a driving rod 32 is joined to theouter end of the horizontal arm portion of one of these lifter pins 28.The driving rod 32 extends downward through a hole formed through thebottom wall of the process vessel 18 such that the lower end of thedriving rod 32 is joined to an actuator 38 positioned below the processvessel 18. Of course, the driving rod 32 can be moved in a verticaldirection by the actuator 38. If the driving rod 32 is moved upward, thethree lifter pins 28 are also moved upward such that the upper portionsof the vertical arm portions of these pins 28 project upward throughlifter pin holes 34 extending through the susceptor 26. It follows thatthe wafer W disposed on the susceptor 26 is moved upward away from thetable 26. To the contrary, if the driving rod 32 is moved downward, thewafer W supported on the upper ends of the lifter pins 28 is also moveddownward so as to be disposed again on the susceptor 26.

A shrinkable bellows 36 is mounted to surround that portion of thedriving rod 32 which is positioned between the lower surface of thebottom plate of the process vessel 18 and the upper surface of theactuator 38 so as to hermetically seal the process vessel 18.

A clamp ring 40 made of a ceramic material and acting as a fixing meansof the wafer W is arranged above the circumferential outer region of thesusceptor 26. To be more specific, the peripheral portion of the wafer Wis pressed from above by the clamp ring 40 against the upper surface ofthe susceptor 26 so as to make the wafer W stationary. As apparent fromthe drawing, the clamp ring 40 is concentric with the susceptor 26 andis fixed to the upper ends of three ring arms 42 each extending looselyin a vertical direction through the horizontal arm portion of thesupport member 24. It is desirable for the ring arm 42 to be formed of aheat ray-transmitting material such as quartz. The lower end of the ringarm 42 is joined to the lifter pin 28. As a result, the clamp ring 40 ismoved in a vertical direction via the ring arms 42 in accordance with avertical movement of the lifter pins 28. Compression coil springs 44 areinterposed between the lower surface of the horizontal arm portion ofthe support member 24 and the upper surface of the horizontal armportion of the lifter pins 28, with the result that the clamp ring 40 isurged downward so as to permit the wafer W to be clamped without fail.It is desirable for lower portions of the ring arms 42 to be insertedinto these coil springs 44, as shown in the drawing. It is alsodesirable for these lifter pins 28 and support member 24 to be formed ofa heat ray-transmitting material such as quartz.

A circular opening concentric with the susceptor 26 is formed in acentral portion of the bottom wall of the process vessel 18. Thecircular opening is positioned right under the susceptor 26 and ishermetically sealed with a transmitting window 46 made of a heatray-transmitting material such as quartz. Further, a box-shaped heatinghousing 48 is mounted to the lower surface of the bottom wall of theprocess vessel 18 in a manner to surround the transmitting window 46. Aheating means, e.g., a plurality of heating lamps 50 (i.e., halogenlamps), is mounted within the heating housing 48 such that the heatingmeans is mounted to an upper surface of a rotatable plate 52 which alsoacts as a reflecting mirror. The rotatable plate 52 is joined to arotary shaft of a motor 54 disposed at a bottom portion of the heatinghousing 48. As a result, the rotatable plate 52 is rotated by the motor54 in a direction denoted by an arrow shown in FIG. 1. It is desirablefor the rotatable plate 52 to be rotated coaxially with the susceptor26. The heat rays emitted from these heating lamps 50 are transmittedthrough the transmitting window 46 so as to irradiate and heat the lowersurface of the susceptor 26. As a result, the wafer W disposed on thesusceptor can be rapidly heated by heat conduction to a predeterminedtemperature.

A cooling air inlet port 56 and a cooling air outlet port 58 are formedon the mutually facing side walls of the heating housing 48. A coolingair is introduced through the cooling air inlet port 56 into the heatinghousing 48 so as to cool the inner space of the heating housing 48 andthe transmitting window 46. Then the air warmed within the heatingchamber is discharged to the outside through the cooling air outlet port58.

A ring-like flow regulating plate 62 having a large number of flowregulating holes 60 formed therethrough is arranged to surround theouter circumferential surface of the susceptor 26. The ring-like flowregulating plate 62 is arranged horizontal and concentric with thesusceptor and is positioned near the circumferential inner surface ofthe process vessel 18. Also, the flow regulating plate 62 is heldbetween the upper outer circumferential surface of a cylindrical supportcolumn 64 and the inner circumferential side wall surface of the processvessel 18. The lower end of the support column is fixed to the bottomwall of the process vessel 18. It should be noted that an upper innercircumferential surface of the support column 64 is stepped to form astepped portion projecting inward. The stepped portion is formed overthe entire circumferential region of the support column. An outercircumferential portion of a ring-like attachment member 66 made ofquartz is mounted to the stepped portion of the support column. Theattachment member 66, which is positioned concentric with the supportcolumn 64, serves to partition the inner space of the process vessel 18into an upper chamber and a lower chamber so as to suppress the flow ofthe process gas into the lower chamber below the susceptor 26 as much aspossible. It is possible to mount a water-cooling jacket (not shown) inthe upper chamber positioned above the support column 64. A coolingwater is circulated within the jacket so as to cool mainly the freespace on the side of the flow regulating plate 62. A plurality ofexhaust ports 68 are formed through those portions of the bottom wall ofthe process vessel 18 which are positioned below the flow regulatingplate 62. These exhaust ports 68 are equidistantly arranged in thecircumferential direction. An exhaust passageway 70 connected to avacuum pump (not shown) is connected to each of the exhaust ports 68. Asa result, the atmosphere within the process vessel 18 can be evacuatedso as to maintain the inner pressure of the vessel 18 at a vacuum of,for example, 100 Torr to 10⁻⁶ Torr.

A pressure release valve (not shown) operable at a differential pressureof about, for example, 0.1 kg/cm² can be mounted in the support column64. The pressure release valve is opened when the inner pressure of agas chamber below the susceptor 26 is excessively increased so as torelease partly the gas through the exhaust port via a free space formedbetween the support column and the inner circumferential surface of theprocess vessel. It follows that the inner pressure of the gas chamber isprevented from being excessively increased by an inert gas flowingbackward into the process vessel.

A shower head section 72 is provided in that region of a ceiling portionof the process vessel 18 which faces the susceptor 26. Gases such as afilm-forming gas (process gas) and a cleaning gas are introduced intothe process chamber 20 through the shower head section 72. To be morespecific, the shower head section 72 comprises a head body 74 made of,for example, aluminum and shaped like a cylindrical box, and a gas inletport 76 provided in a central portion of the ceiling of the head body74.

A gas ejection wall 78 constitutes the bottom wall of the head body 74.A large number of gas ejection holes 80 for releasing the gas suppliedinto the head body 74 are uniformly arranged over the entire region ofthe gas ejection wall 78, with the result that the gas is uniformlyreleased over the entire surface of the wafer W supported on the table26. The gas ejection wall 78 has a diameter of, for example, 316 mm.

On the other hand, the gas ejection hole 80 has a diameter of about 1mm. These gas ejection holes 80 are formed at a density of about 10holes/cm² in a central portion, having a diameter of 230 mm, of the gasejection wall. These values in respect of the diameters of the gasejection wall and the gas ejection hole and the density of the gasejection holes 80 are substantially equal to those in the conventionalshower head section.

Two uniform dispersion plates, i.e., upper and lower dispersion plates82 and 86, respectively, are superposed one upon the other apredetermined distance apart from each other within the head body 74 soas to form a plurality of diffusion chambers within the head body 74.The presence of these dispersion plates permits the apparatus of thepresent invention to produce a prominent effect. The upper dispersionplate 82 is provided with a very small number of gas spurting holes,e.g., only one gas passing hole 84 or several gas passing holes. In theembodiment shown in FIG. 1, the upper dispersion plate 82 is providedwith only one gas passing hole 84 having a diameter of about 1.5 mm. Informing a plurality of gas passing holes through the upper dispersionplate 82, it is possible for all holes to have the same diameter. It isalso possible for some of these holes to have the same diameter. Forexample, it is possible to form a relatively large single gas passinghole 84 in the center and relatively small six gas passing holes 84arranged to surround the central gas passing hole.

A large number of very small dispersion holes 88 are formed uniformlythrough the lower uniform dispersion plate 86. The diameter of thedispersion hole 88 and the distribution density of these dispersionholes are determined to permit a gas stream to be supplied uniformlyover the entire region of the process chamber 20 in each of the caseswhere film-forming gases are supplied from the shower head section 72for carrying out the reaction rate-limiting reaction and wherefilm-forming gases are supplied for carrying out the gas supplyrate-limiting reaction. To be more specific, where the dispersion hole88 has an unduly large diameter, it is certainly possible to carry outthe reaction rate-limiting reaction by suitably changing thedistribution density of the dispersion holes 88. However, it isimpossible to control as desired the reactant gas supply rate-limitingreaction. To the contrary, where the diameter of the dispersion hole 88is excessively small, it is certainly possible to carry out the reactantgas supply rate-limiting reaction by increasing the distribution densityof the dispersion holes 88. However, it is impossible to control asdesired the reaction rate-limiting reaction. In other words, in order toenable the apparatus to carry out satisfactorily each of the reactantgas supply rate-limiting reaction and the reaction rate-limitingreaction, it is necessary to choose appropriately the diameter of thedispersion hole 88 and the distribution density of these dispersionholes 88.

In the embodiment shown in the drawing, the diameter (or inner diameter)of the dispersion hole 88 should be about at most 0.7 mm. On the otherhand, the dispersion holes 88 should be formed at a distribution densityof at least 0.3 hole/cm². The lower limit in the diameter of thedispersion hole 88 is not particularly limited in the present invention,as far as it is possible for the gas to flow smoothly through the hole88. However, the lower limit in the diameter of the dispersion hole isdetermined by the performance of the punching tool used. Where, forexample, the lower uniform dispersion plate 86 has a thickness of about10 mm, the lower limit in the diameter of the dispersion hole 88 shouldbe about 0.2 mm.

On the other hand, the upper limit in the distribution density of thedispersion holes 88 is not particularly limited in the presentinvention, as far as adjacent holes are not joined to each other.

Preferably, the diameter of the dispersion hole should fall within arange of between about 0.1 mm and 0.7 mm.

Also, the distribution density of the dispersion holes should fallwithin a range of between 0.3 hole/cm² and 1.0 hole/cm². Where, forexample, the uniform dispersion plate 86 has a diameter of about 30 cm,it is desirable to form uniformly about 190 dispersion holes each havinga diameter of 0.65 mm. FIG. 2 is a plan view partly showing the uniformdispersion plate 86 of this construction. The uniform dispersion plateof this construction permits uniformly supplying gases over the entireregion of the reaction chamber in each of the cases where film-forminggases are supplied for carrying out the reaction rate-limiting reactionand the case where the film-forming gases are supplied for carrying outthe reactant gas supply rate-limiting reaction.

A single gas passageway 90 is connected at one end to the gas inlet port76 of the shower head section 72 and at the other end portions tovarious film-forming gas sources and cleaning gas sources via variousbranched passageways 92. As shown in FIG. 1, the gas passageway 90 isconnected to an Ar gas source 94 for storing an Ar gas used as a carriergas, to an SiH₄ source 96 for storing an SiH₄ gas used as a film-forminggas, to an SiH₂ Cl₂ source 98 for storing an SiH₂ gas used as afilm-forming gas, to a WF₆ source 100, to a PH₃ source 102 for storing aPH₃ gas used as a doping gas, and to a ClF₃ source 104 for storing aClF₃ gas used as a cleaning gas. A flow rate control valve 106 such as amass flow controller and an ordinary valve 108 are connected in seriesto each of the branched passageways 92. In the embodiment shown in thedrawing, the ordinary valve 108 is interposed between the gas source andthe flow rate control valve 106, though it is possible to reverse thepositions of these valves 106 and 108.

Further, an opening for transferring the wafer W into or out of theprocess chamber 20 is formed through one side wall of the process vessel18. The particular opening communicates with a load lock chamber 110 viaa gate valve G. The inner space of the load lock chamber 110 is heldvacuum, and the wafer which is to be processed is temporarily stored inthe load lock chamber.

The apparatus of the construction described above is used for working amethod of the present invention for forming laminated thin films or amulti-layer construction. Specifically, FIG. 3 is a flow chart showingthe process steps employed in the method of the present invention. Asdescribed previously, the present invention is featured in that, forforming a plurality of different kinds of films within a single processchamber, the process gases and the carrier gas are introduced into theprocess chamber as a pretreatment which is carried out without disposingthe wafer W on the table 26 so as to pre-coating in advance the innersurfaces of, for example, the process chamber with films of thecompositions equal to those of the films formed on the wafer W. Thepresent invention is also featured in that, after formation of thefilms, a surface treatment is carried out with a silane-series gas inthe presence of the wafer W. This embodiment covers the case where thepolysilicon layer 10 doped with phosphorus and the tungsten silicidelayer 12 are formed in succession as shown in FIG. 9A.

In the first step, a pretreatment is performed before the wafer W isloaded in the process chamber 20 (step S1). In this pretreatment, thegases, which are to be introduced into the process chamber in thesubsequent step of forming desired films on the wafer and in theafter-treatment step, are introduced into the process chamber so as toform pre-coating laminated films on the inner surfaces of the wall andthe internal structure of the process vessel 18. As describedpreviously, these pre-coating films are intended to make the conditionswithin the process vessel such as the thermal reflectance and emissivityequal to those in the step of forming the desired laminated films on thewafer W so as to improve the reproducibility of the film formation onthe wafer.

In this pretreatment step, the process chamber is evacuated first to apredetermined vacuum level. At the same time, the inner temperature ofthe process vessel is elevated to 500° C. to 800° C. Under theseconditions, predetermined amounts of gases equal to those used forforming a polysilicon film doped with phosphorus on the wafer W areintroduced into the process vessel so as to form a pre-coatingpolysilicon film doped with phosphorus on the inner surface of theprocess chamber 20 and on the surfaces of the internal structure withinthe process vessel 18 such as the clamp ring 40 and the flow regulatingplate 62. In this embodiment, predetermined amounts of Ar gas (carriergas), SiH₄ gas (reactant gas), and PH₃ gas (dopant gas) are introducedinto the process chamber 18 for forming the pre-coating polysilicon filmnoted above. Incidentally, the PH₃ gas used as a dopant gas scarcelyaffects, for example, the thermal reflectance, making it possible toomit the PH₃ gas supply in the pretreatment step.

After formation of the pre-coating polysilicon film, the gases remainingwithin the process vessel 18 are discharged by vacuum suction from theprocess vessel. Then, predetermined amounts of gases equal to those usedfor forming a tungsten silicide (WSi_(x)) film on the wafer W areintroduced into the process vessel so as to form a tungsten silicidefilm on the pre-coating polysilicon film doped with phosphorus. In thisembodiment, predetermined amounts of Ar gas, WF₆ gas, and SiH₂ Cl₂ gasare introduced into the process chamber 18 for forming the tungstensilicide film noted above on the pre-coating polysilicon film formed inadvance. It is desirable for the pre-coating amounts for each of thedoped polysilicon film and the tungsten silicide film to be somewhatlarger than those for actually forming the doped polysilicon film andthe tungsten silicide film on the wafer W so as to improve thereproducibility of forming these films on other wafers W.

In this embodiment, a silane-series gas is introduced into the processvessel 18 in the after-treatment step for allowing silicon to bedeposited on the surface of the tungsten silicide film formed on thewafer. In this connection, a predetermined amount of an SiH₄ gas used asa silane-series gas is introduced in the pretreatment step into theprocess chamber together with an Ar gas used as a carrier gas so as topermit silicon to be deposited in a small amount on the surface of thepre-coated tungsten silicide film. Incidentally, it is also possible touse an SiH₂ Cl₂ gas as a silane-series gas.

After the pretreatment carried out in the absence of the wafer W, thewafer is loaded in the process chamber 20 (step S2). To be morespecific, an untreated semiconductor wafer W housed in the load lockchamber 110 is transferred through the gate valve G into the processchamber 20. At the same time, the lifter pins 28 are pushed up so as topermit the wafer W to be delivered toward the lifter pins 28. Then, thewafer W is disposed on the susceptor 26, followed by moving furtherdownward the push up rod 32. As a result, the clamp ring 40 is allowedto be pressed against a peripheral portion of the wafer W. It followsthat the wafer W is fixed to the susceptor 26. Incidentally, the term"untreated semiconductor wafer W" noted above represents a wafer havingthe gate oxide film 6, which is shown in FIG. 9A, formed in advance inanother process furnace.

When the wafer loading in the process chamber is finished in thisfashion, the step of actually forming a polysilicon film on the gateoxide film 6 is carried out (step S3). In the first step, formed is apolysilicon film doped with phosphorus. In this step, the heating lamps50 arranged within the heating housing 48 are rotated so as to achieve aheat energy emission uniformly, while evacuating the process chamber 20by means of vacuum suction. The heat rays thus emitted are transmittedthrough the transmitting window 46 and, then, through other members ofthe apparatus such as the support member 30 made of quartz so as toirradiate the back surface of the susceptor 26. As described previously,the susceptor 26 is very thin, i.e., about 1 to 5 mm, with the resultthat the table 26 is rapidly heated. Naturally, the wafer W disposed onthe susceptor 26 is also heated rapidly to a predetermined temperature.

If the wafer W is heated to a process temperature of, for example, 700°C., the SiH₄ (silane) gas and the PH₃ (phosphine) gas are introducedtogether with the carrier gas of Ar gas into the process chamber 20through the shower head section 72. These silane and phosphine gases aresupplied at rates of about 150 sccm and about 400 sccm, respectively.

A predetermined chemical reaction takes place within the process chamber20 between the silane and phosphine gases so as to form a polysiliconlayer 10 doped with phosphorus (P) acting as an impurity of oneconductivity type on the gate oxide film 6 formed on the wafer W, asshown in FIG. 9A. Incidentally, it is also possible to use dopants otherthan phosphorus such as As (arsenic), Sb (antimony) and B (boron). Inorder to obtain the doped polysilicon layer 10 having a predeterminedthickness, the film-forming operation described above is performed forabout, for example, one minute. Also, the process pressure in this stepis about 10 Torr.

After formation of the doped polysilicon layer 10, a tungsten silicidefilm is formed on the polysilicon layer 10 as follows. In the firststep, the supply of the PH₃ gas and SiH₄ gas is stopped. Under thiscondition, an Ar gas is allowed to flow through the process chamber 20so as to purge the phosphine gas from within the process chamber 20. Atthe same time, the power supplied to the heating lamps 50 is controlledso as to slightly lower the temperature of the wafer W to the processtemperature of the tungsten silicide, e.g., about 600° C. The purgingwith the argon gas is performed for several minutes. In this case, it ispossible to set the process temperature of tungsten silicide at a valueequal to that of polysilicon.

When the process temperature is reached, the reactant gases of SiH₂ Cl₂gas and WF₆ gas are supplied into the process chamber 20 together with acarrier gas of Ar gas so as to form a tungsten silicide film on thepolysilicon film. The flow rates of these SiH₂ Cl₂ gas and WF₆ gasshould be about 200 sccm and about 10 sccm, respectively. It is possibleto use SiH₄ gas, etc. in place of the SiH₂ Cl₂ gas. Also, an N₂ gas orHe gas can be used in place of the argon gas used as a carrier gas.

A predetermined chemical reaction takes place within the process chamber20 between the reactant gases so as to form the tungsten silicide layer12 shown in FIG. 9A. In order to obtain the tungsten silicide layerhaving a predetermined thickness, the film-forming reaction is carriedout for about, for example, 2 minutes. The process pressure in this stepshould be about 1 Torr.

It should be noted that the film-forming reaction for forming thepolysilicon layer or film 10 doped with phosphorus is a reactionrate-limiting reaction. On the other hand, the film-forming reaction forforming the tungsten silicide layer or film 12 is a reactant supplyrate-limiting reaction. In the apparatus of the present invention,however, the diameter of the dispersion hole 88 formed through theuniform dispersion plate 86, which is arranged within the shower headsection 72, and the distribution density of the dispersion holes 88 areset appropriately, making it possible to supply the reactant gases andthe carrier gas into the process chamber uniformly over the entireregion of the process chamber 20 for each of the reaction rate-limitingreaction and the reactant supply rate-limiting reaction. It follows thateach of these polysilicon layer 10 and tungsten silicide layer 12 can beformed in a uniform thickness over the entire region.

After formation of the tungsten silicide layer 12, an after-treatment iscarried out with the wafer W left disposed on the susceptor 26 (stepS4). In the first step, the inner atmosphere of the process vessel 18 isevacuated by vacuum suction. Under this condition, a silane-series gas,e.g., SiH₄ gas, is supplied into the process chamber 20 for a shorttime, e.g., about 60 seconds, while substantially maintaining thetemperature in the step of forming the tungsten silicide layer 12, e.g.,600° to 700° C., so as to deposit silicon slightly on the surface of thetungsten silicide layer. The silicon deposition amount is controlled toform a thin silicon film on the tungsten silicide layer 12 or to allowsilicon flakes to be attached only slightly to the tungsten silicidelayer not to form a continuous silicon film. The deposited siliconpermits moderating the stress within the tungsten silicide layer so asto improve the bonding strength between the polysilicon layer 10 and thetungsten silicide layer 12, as described herein later. Also, thedeposited silicon makes it possible to prevent the tungsten silicidelayer 12 from being attacked by oxygen in the subsequent heat treatmentstep.

After completion of the after-treatment described above, the heatinglamps 50 are turned off so as to lower the wafer temperature to, forexample, about 300° C. adapted for the wafer transfer while purging thegases remaining inside the process vessel 18 with an argon gas. Then,the gate valve G is opened so as to unload the treated wafer W out ofthe process vessel 18 (step S5), followed by loading an untreated waferinto the process chamber as described previously ("NO" in step S6). Inthis case, the film-forming step S3 and the after-treatment step S4 arecontinuously applied to the new untreated wafer as described previously.The continuous processing is applied to one lot consisting of, forexample, 25 wafers.

After the unloading of a predetermined number of wafers, e.g., 25 wafers("YES" in step S6), the process vessel is treated with a cleaning gas inorder to remove the films remaining in small amounts on the surfaces ofthe inner wall of the process chamber and the internal structure withinthe process vessel 18 (step S7). To be more specific, a ClF₃ gas used asa cleaning gas is introduced into the process vessel. Under thiscondition, the process chamber is heated to, for example, about 200° C.so as to carry out the cleaning treatment for several minutes. As aresult, the undesired films attached to the inner wall of the processchamber can be removed, making it possible to suppress problems such asthe particle generation which is caused by the peeling of the attachedfilms noted above. The cleaning treatment is performed in view of theamount of the films attached to the inner wall of the process chamber,etc. For example, the cleaning treatment is performed every time theseries of treatments described above are applied to a single wafer. Inshort, it suffices to determine the number of times of applying theseries of treatments in view of the through-put and the amount of theparticle generation. It should be noted that a fluorine-containing gaspermits effectively removing each of polysilicon and tungsten silicide.Therefore, it is desirable to use, for example, a ClF₃ -series gas as acleaning gas, though the cleaning gas used in the present invention neednot be limited to ClF₃ gas.

After completion of the cleaning step, an after-treatment after thecleaning treatment is performed (step S8). In this step, a silane-seriesgas is supplied into the process chamber so as to promote discharge ofthe halogen gas used in the cleaning step and, thus, to improve theadhesion of pre-coating films which are to be formed in the subsequentpretreatment step.

To be more specific, after discharge of the cleaning gas, thetemperature within the process chamber is elevated to, for example,about 600° C. Under this condition, a heat treatment is carried out forabout one minute while allowing an SiH₄ gas to flow into the processchamber at a predetermined flow rate. As a result, the halogen gasattached to the inner wall of the process chamber and to the surface ofthe internal structure is reduced with the silane gas. It follows thatthe discharge of the halogen gas is promoted. It is possible to use anSiH₂ Cl₂ gas in place of the silane (SiH₄) gas.

In the method of the present invention, one process cycle consists ofsteps S1 to S8 described above.

As described above, the method of the present invention is featured inthat the tungsten silicide layer 12 is formed immediately afterformation of the polysilicon layer 10 doped with phosphorus within thesame process chamber. In other words, the wafer W need not be taken outof the process chamber after formation of the polysilicon film.Naturally, it is substantially impossible for a native oxide film to beformed on the surface of the polysilicon layer 10. It follows that theresistance of the gate electrode can be lowered. Also, the electricalcharacteristics can be improved markedly.

It should also be noted that, in the present invention, pre-coatinglaminated films are formed on the inner wall of the process chamber andon the surface of the internal structure before formation of thepolysilicon layer 10 and tungsten silicide layer 12 on the wafer W. Itis important to note that these pre-coating films are formed of thematerials equal to those of the layers 10 and 12 formed on the wafer W.Also, a film of the gas used in the after-treatment is included in thepre-coating films. It follows that the process chamber is thermallystabilized so as to maintain a high reproducibility of the layers 10 and12 formed on the wafer W.

Further, an after-treatment is applied to the wafer after formation ofthe layers 10 and 12 so as to allow silicon continuous or discontinuousthin films to be deposited on the surface of the tungsten silicide layer12. As a result, the stress within the tungsten silicide layer 12 ismoderated. Also, the layer 12 can be prevented from being attacked byoxygen in the subsequent heat treating step.

The pre-coating films formed in the pretreatment step produce prominenteffects. Specifically, FIG. 4 is a graph showing the changes in thesheet resistance of the gate electrode included in each wafer, coveringthe case where a film is formed on the surface of each of thesemiconductor wafers within first and second lots, each lot consistingof 25 wafers. In this experiment, pre-coating films were formed in thepretreatment immediately before each of the first lot and the secondlot. As shown in FIG. 4, a difference between the maximum and minimumvalues of the sheet resistance was found to be about 5 Ω/□. In otherwords, a deviation in the sheet resistance among the wafers was found tobe about 3%, supporting a markedly high reproducibility.

As described previously, an after-treatment step is carried outimmediately after the step of forming the polysilicon layer 10 and thetungsten silicide layer 12 so as to deposit continuous or discontinuoussilicon thin films on the surface of the layer 12. The silicondeposition produces a prominent effect as shown in FIG. 5. To reiterate,FIG. 5 is a graph showing the degree of oxygen diffusion into the formedfilm, covering the cases where silicon is attached, and not attached, tothe upper surface of the tungsten silicide layer 12. In the graph ofFIG. 5, the depth from the upper surface of the gate electrode towardthe wafer is plotted on the abscissa, with the oxygen amount beingplotted on the ordinate indirectly. In this experiment, the oxygenattack was performed by heating the wafer to about 90° C.

As apparent from the graph of FIG. 5, the silicon deposition by theafter-treatment on the surface of the tungsten silicide layer 12, whichis denoted by a broken line, permits decreasing the oxygen amount in thetungsten silicide (WSi_(x)) layer, compared with the non-deposition,which is denoted by a solid line. In other words, the oxygen diffusionis suppressed by the silicon deposition.

FIG. 6 is a graph showing another effect produced by the after-treatmentstep in respect of moderation of stress remaining within the tungstensilicide layer. In the graph of FIG. 6, the supply time of the silanegas into the process chamber and the supply time of the dichlorosilane(SiH₂ Cl₂) gas into the process chamber are plotted on the abscissa,with the stress being plotted on the ordinate. In this after-treatmentstep, the silane gas flow rate was set at SiH₄ /Ar=500/400 sccm, thedichlorosilane gas flow rate was set at SiH₂ Cl₂ /Ar=150/350 sccm, andthe process pressure was set at 0.7 Torr.

As apparent from the graph of FIG. 6, the stress within the film was ashigh as 1.30×10¹⁰ dyn/cm² in the case where the after-treatment was notperformed (after-treating time of zero second). However, the stress islowered sufficiently by the after-treatment in each of theafter-treatments with the silane gas and with the dichlorosilane gas. Itfollows that the bonding strength between the polysilicon layer 10 andthe tungsten silicide layer 12 can be increased by the after-treatment.It should also be noted that a satisfactory effect can be produced inthe case of using either a silane gas or a dichlorosilane gas in theafter-treatment.

As described previously, another after-treatment, i.e., treatment with asilane-series gas, is performed after the cleaning step so as to removehalogen elements attached to the inner wall of the process chamber tothe surface of the internal structure of the process vessel 18. FIG. 7is a graph showing the effect produced by this second after-treatmentwith a silane-series gas in respect of the remaining amount of a halogenelement (chlorine) on the surface of the tungsten silicide (WSi_(x))film. In this experiment, silane (SiH₄) was used as the silane-seriesgas. Curve B in FIG. 7 shows the case of performing said anotherafter-treatment after the cleaning step, with curve A denoting the caseof not performing the particular after-treatment with a silane-seriesgas. As apparent from FIG. 7, the after-treatment with a silane gas iseffective for suppressing the remaining halogen amount so as to suppressadverse effects given by the halogen element on the tungsten silicidelayer.

The particular after-treatment with a silane-series gas produces anadditional effect. Specifically, it is possible to facilitate formationof pre-coating films in the pretreatment on the inner wall of theprocess chamber 20 and on the surface of the internal structure in theprocess vessel 18, as shown in FIG. 8. In the graph of FIG. 8, the timefor the pre-coating films to reach a predetermined thickness is plottedon the ordinate. Bars P and Q in the graph of FIG. 8 cover the cases ofnot applying the after-treatment with the silane gas, and are directedto the reactant gas supply-rate limiting reaction and the reactionrate-limiting reaction, respectively. On the other hand, bar R coversthe case of applying the after-treatment with the silane gas. Asapparent from FIG. 8, the after-treatment with the silane gas iseffective for shortening the time required for forming pre-coating filmsof a predetermined thickness in each of the reactant gas supplyrate-limiting reaction and the reaction rate-limiting reaction, leadingto shortening of the entire processing time. It should be noted that thepurging with a silane gas in the pretreatment makes it possible toshorten the incubation time during which a film is not formed, leadingto the shortened pre-coating time.

As described previously, the shower head section 72 included in theapparatus of the present invention is of a two-layer structure. Theparticular construction is effective for continuous formation of films,as shown in Table 1. In this experiment, the diameters and numbers ofthe dispersion holes in each of the upper dispersion plate 82 and theuniform dispersion plate 82 were changed in various fashions in anattempt to look into the differences in formation of tungsten silicidefilm.

                                      TABLE 1                                     __________________________________________________________________________    Comparative  Comparative                                                                          Comparative                                               Example 1      Example 2                                                                            Example 3                                                                            Example 1                                                                              Example 2                               __________________________________________________________________________    Upper 13 mm φ ×                                                                  3 mm φ ×                                                                   1.5 mm φ ×                                                                 1.5 mm φ ×                                                                   1.5 mm φ ×                      dispersion                                                                          8 holes                                                                                8 holes                                                                              1 hole                                                                                1 hole                                                                                 1 hole                                 plate                                                                         Uniform                                                                             4 mm φ × 188"                                                              4 mm φ × 188"                                                              4 mm φ × 188"                                                              0.65 mm φ × 188"                                                              0.5 mm φ × 722"                dispersion                                                                          (0.3 hole/cm.sup.2)                                                                   (0.3 hole/cm.sup.2)                                                                  (0.3 hole/cm.sup.2)                                                                  (0.3 hole/cm.sup.2)                               plate                                                                         Formation                                                                           Formed on                                                                              Considerably                                                                         8%      2%       2%                                     of tungsten                                                                         edge portion                                                                           thin in                                                        silicide                                                                            alone     central                                                       film            portion                                                       __________________________________________________________________________

The gas ejection wall 78 used in this experiment, which had a diameterof about 316 mm, was similar to a conventional gas ejection wall in thediameter of the gas ejection hole 80 and the distribution density of theholes 80. For example, the diameter of the hole 80 was 1 mm, and 4397holes were arranged within a circular area having a diameter of 230 mm,i.e., about 10 holes/cm². Also, the diameter of the dispersion plate wasset at about 260 mm. It should be noted that, in order to supply thegases uniformly over the entire region of the process chamber 20, theconstruction of the uniform dispersion plate 86 arranged below the upperdispersion plate 82 is very important. Therefore, the construction ofthe uniform dispersion plate 86 was changed in various fashions in thisexperiment.

As shown in Table 1, the diameters of the holes formed in the upperdispersion plates for Comparative Examples 1 and 2 were considerablylarge, i.e., 13 mm and 3 mm, respectively. In addition, a large numberof holes, i.e., 8 holes, were formed in the upper dispersion plate ineach of Comparative Examples 1 and 2. In each of these cases, a tungstensilicide layer, which is formed by a reactant supply rate-limitingreaction, is formed on an edge portion alone of the wafer.

Comparative Example 3 was set equal to Comparative Examples 1 and 2 inthe diameter and the number of holes formed through the uniformdispersion plate 86. In Comparative Example 3, however, the diameter ofthe hole formed through the upper dispersion plate 82 was small, i.e.,about 1.5 mm. In addition, only one hole was formed through the upperdispersion plate 82. A tungsten silicide layer was certainly formed overthe entire region of the wafer including the central portion inComparative Example 3. However, the tungsten silicide layer in thecentral portion was found to be considerably thinner than that in theedge portion.

On the other hand, the upper dispersion plate 82 equal to that used inComparative Example 3 was used in each of Examples 1 and 2 of thepresent invention. In Example 1, however, the diameter of the holeformed through the uniform dispersion plate 86 was set at 0.65 mm. Also,188 holes were formed through the uniform dispersion plate 86 (about 0.3hole/cm²). Further, in Example 2, the diameter of the hole 88 formedthrough the uniform dispersion plate 86 was set at 0.5 mm. Also, 722holes were formed through the uniform dispersion plate 86 (about 1.4hole/cm²). As shown in Table 1, the uniformity in thickness of thetungsten silicide layer over the entire region of the wafer was found tobe only about 2% in each of Examples 1 and 2, supporting that it is veryimportant to determine appropriately the diameter and the number ofdispersion holes 88 formed through the uniform dispersion plate asdefined in the present invention.

In the embodiment described above, a semiconductor wafer is used as anobject to be processed. However, it is also possible to employ thetechnical idea of the present invention for the processing of otherobjects such as a glass substrate and an LCD substrate.

Also, in the embodiment described above, the first gas compositionsupplied into the process chamber in the pretreatment for forming afirst pre-coating film, i.e., polysilicon film doped with phosphorus,was equal to the first gas composition supplied into the process chamberfor forming a first layer, i.e., the polysilicon layer 10 doped withphosphorus, on the wafer. However, it is not absolutely necessary to usethe gases of the same composition for forming the pre-coating film andpolysilicon layer 10 on the wafer. For example, it is possible to omitthe use of a PH₃ gas for forming the first pre-coating film as alreadydescribed herein before. In this case, the pre-coating film (polysiliconfilm) formed in the pretreatment is not doped with phosphorus, thoughthe polysilicon layer 10 formed in the subsequent step on the wafer W isdoped with phosphorus. It follows that, strictly speaking, the firstpre-coating film formed in the pretreatment and the polysilicon layer 10formed on the wafer W are not exactly equal to each other. However, itis practically reasonable to understand that these pre-coating film andthe polysilicon layer consist mainly of the same composition.

On the other hand, it is possible for the second gas composition used inthe pretreatment for forming the second pre-coating film, i.e., tungstensilicide film, to be different from the second gas composition used forforming the tungsten silicide layer 12. For example, it is possible touse an SiH₂ Cl₂ gas for forming the second pre-coating film and an SiH₄gas for forming the tungsten silicide layer 12 on the wafer. Of course,such a small difference in respect of the presence or absence and thekind of the impurity is acceptable, as far as the second pre-coatingfilm and the tungsten silicide layer 12 formed on the wafer aresubstantially equal to each other in composition.

In the embodiment described above, the gate electrode is of two layerstructure consisting of a polysilicon layer and a tungsten silicidelayer formed on the polysilicon layer. However, the construction of thegate electrode is not limited to that noted above. Of course, the gateelectrode may be formed of three or more layers in place of the gateelectrode of the two layer structure.

As described above, the film-forming method and apparatus of the presentinvention produce prominent effects. To reiterate, the method of thepresent invention is featured in that, in preparation for consecutiveformation of a plurality of films on a wafer within the process chamber,gas compositions are supplied in advance into the process chamber forforming a plurality of pre-coating films on the inner surface of theprocess chamber and on the surface of the internal structure of theprocess vessel so as to stabilize the thermal reflectance, thermalemissivity, etc. on the inner surface of the process chamber and on thesurface of the internal structure of the process vessel. As a result,the laminate structure consisting of a polysilicon layer and a tungstensilicide layer can be consecutively formed on a wafer with a highreproducibility.

Also, after the consecutive film-forming operations to form the laminatestructure noted above on a wafer, an after-treatment is performed inwhich a silane-series gas is allowed to flow through the process vessel.As a result, the stress remaining in the upper layer of the laminatestructure can be moderated so as to improve the bonding strength betweenthe upper and lower layers of the laminate structure. In addition, theparticular after-treatment is effective for inhibiting an oxygendiffusion into the upper layer of the laminate structure in thesubsequent heat treating step.

It should also be noted that after the consecutive film-formingoperations to form the laminate structure noted above on a wafer, theprocess chamber is purged with a halogen gas, e.g., afluorine-containing gas, followed by allowing a silane-series gas toflow through the process vessel. As a result, the halogen gas remainingwithin the process vessel can be effectively released out of the processchamber. Also, the treatment with the silane-series gas noted above iseffective for promoting the pre-coating film formation during thepretreatment for the subsequent operations for forming the films on anew wafer.

Further, in the film-forming apparatus of the present invention, auniform dispersion plate is arranged within a shower head section of theapparatus. What should be noted is that a large number of dispersionholes having a small diameter are formed through the uniform dispersionplate at a high distribution density, making it possible to supply thegases uniformly over the entire region of the process chamber forforming a film in each of the reactant gas supply rate-limiting reactionand the reaction rate-limiting reaction. It follows that a plurality ofdifferent kinds of films can be successively formed within the sameprocess chamber such that the formed films are uniform in thickness overthe entire region of the wafer or the like.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method of forming laminated layers, comprisingthe steps of:(A) forming pre-coating films in a pretreatment bysupplying a first series of film-forming gases into a process chamberwhile heating said process chamber so as to form a first pre-coatingfilm consisting essentially of polycrystalline silicon doped withphosphorus on an inner surface of said process chamber, followed bysupplying a second series of film-forming gases into the process chamberto form a second pre-coating film consisting essentially of tungstensilicide on said first pre-coating film; (B) loading an object to beprocessed into the process chamber; (C) supplying the first series ofgases into the process chamber while heating the process chamber so asto form a first layer consisting essentially of polycrystalline silicondoped with phosphorus on said object, followed by supplying the secondseries of gases into the process chamber so as to form a second layerconsisting essentially of tungsten silicide on said first layer; (D)supplying a silane-series gas into the process chamber to permit siliconmaterial to be deposited on the second layer stacked on the first layer;and (E) unloading the object having the first and second layers formedthereon out of the process vessel.
 2. The method of forming laminatedlayers according to claim 1, further comprising additional steps of:(F)applying said steps (A) to (E) to at least one additional object aftersaid step (E) of unloading the object having the first and second layersformed thereon; (G) supplying a halogen-series cleaning gas into theprocess chamber after step (E) applied to said additional object whileheating the process vessel including the process chamber so as tocleanse the process vessel; and (H) supplying a silane-series gas intothe process chamber while heating the process vessel including theprocess chamber so as to remove the halogen element remaining within theprocess vessel.
 3. The method of forming laminated layers, comprisingthe steps of:(a) forming pre-coating films in a pretreatment bysupplying a first series of film-forming gases into a process chamberwhile heating said process chamber so as to form a first pre-coatingfilm consisting essentially of polycrystalline silicon doped withphosphorus on an inner surface of said process chamber, followed bysupplying a second series of film-forming gases into the process chamberto form a second pre-coating film consisting essentially of tungstensilicide on said first pre-coating film; (b) loading an object to beprocessed into the process chamber; (c) supplying the first series ofgases into the process chamber while heating the process chamber so asto form a first layer consisting essentially of polycrystalline silicondoped with phosphorus on said object, followed by supplying the secondseries of gases into the process chamber so as to form a second layerconsisting essentially of tungsten silicide on said first layer; (d)supplying a silane-series gas into the process chamber to permit siliconmaterial to be deposited on the second layer stacked on the first layer;(e) repeating said steps (a) to (d) a predetermined number of times toobtain a plurality of objects each having a laminate structureconsisting of the first and second layers formed thereon; (f) supplyinga halogen-series cleaning gas into the process chamber after said step(e) while heating the the process chamber so as to cleanse the processchamber; and (g) supplying a silane-series gas into the process chamberwhile heating the process chamber so as to remove the halogen elementremaining within the process chamber.
 4. A method of forming laminatedlayers, comprising the steps of:(a) forming a first pre-coating layerconsisting essentially of polycrystalline silicon on an inner surface ofa process chamber by supplying a silicon-bearing gas into the processchamber; (b) forming a second pre-coating layer consisting essentiallyof tungsten silicide on the first pre-coating layer by supplying a gasbearing tungsten and silicon into the process chamber; (c) placing asubstrate in the process chamber; (d) forming a first layer on thesubstrate by supplying a silicon-bearing gas into the process chamber,said first layer consisting essentially of polycrystalline silicon; and(e) forming a second layer on said first layer by supplying a gasbearing tungsten and silicon into the process chamber, said second layerconsisting essentially of tungsten silicide.
 5. A method according toclaim 4, further comprising:a step between the steps (b) and (c), ofsupplying a silicon-bearing gas into the process chamber to form asilicon layer on the second pre-coating layer; and a step after the step(e), of supplying a silicon-bearing gas into the process chamber to forma silicon layer on the second layer.
 6. A method according to claim 4,further comprising:a step between the steps (b) and (c), of supplying asilicon-bearing gas into the process chamber to form a silicon layer onthe second pre-coating layer.
 7. A method according to claim 4, furthercomprising:a step after the step (e), of supplying a silicon-bearing gasinto the process chamber to form a silicon layer on the second layer. 8.A method according to claim 4, wherein the thickness of the firstpre-coating layer is larger than that of the first layer on thesubstrate and the thickness of the second pre-coating layer is largerthan that of the second layer on the first layer.
 9. A method accordingto claim 4, wherein the first pre-coating layer and/or the first layercontains a dopant.
 10. A method according to claim 9, wherein the dopantis phosphorus, arsenic, antimony or boron.
 11. A method of forminglaminated layers on a substrate, comprising the steps of:(a) placing thesubstrate in a process chamber; (b) forming a first layer on thesubstrate by supplying a silicon-bearing gas into the process chamber,said first layer consisting essentially of polycrystalline silicon; (c)forming a second layer on said first layer by supplying a gas bearingtungsten and silicon into the process chamber, said second layerconsisting essentially of tungsten silicide; and (d) supplying asilicon-bearing gas into the process chamber to form a silicon layerdirectly on the second layer.